This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. During the reporting period, we concluded our studies in the considerable individual variation in the propensity to form a partner preference and to display paternal care among male prairie voles. Unlike most mammalian species, prairie voles (Microtus ochrogaster) are socially monogamous. We concluded that prairie voles are a powerful model species to investigate the neurobiological mechanisms underlying variation in social behavior. We found that the vasopressin V1a receptor (V1aR) plays a central role in both pair bond formation and paternal care in the male prairie vole. Variation in both of these behaviors has been associated with variation in V1aR density in several brain regions, including the lateral septum (LS). We have developed a selective breeding strategy to determine whether V1aR density in the LS is a major source of heritable variation in social behavior. This study concluded in December 2010.